Stop n-cpu by correct process name /w fake

When VIRT_DRIVER=fake, n-cpu processes are numbered (ie, n-cpu-1) in
start_nova. However, this scheme is not taken into account when
stopping nova, resulting in leftover n-cpu processes that fail
grenade's stop-base if USE_SCREEN=False. This special cases for the
fake driver in stop_nova_compute and ensures n-cpu(s) is shutdown
correctly.

Change-Id: Icebece9eadc4e10bb12fe4fdd2fa37d5f3983f66
Close-bug: #1378112
Merge "Stop n-cpu by correct process name /w fake"